Y. Ao, C. Henkel, K. M. Menten, M. A. Requena-Torres, T. Stanke, R. Mauersberger, S. Aalto, S. Muehle, J. Mangum
We have used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the $J_{\rm K_aK_c}$ = 3$_{03}\rightarrow2_{02}$, 3$_{22}\rightarrow2_{21}$, and 3$_{21}\rightarrow2_{20}$ transitions of para-H$_2$CO at 218 GHz simultaneously to determine kinetic temperatures of the dense gas in the Central Molecular Zone (CMZ) of our Galaxy. The map extends over approximately 40${^\prime}$$\times8{^\prime}$ ($\sim100\times$20 pc$^2$) along the galactic plane with a linear resolution of 1.2 pc. The strongest of the three lines, the H$_2$CO (3$_{03}\rightarrow2_{02}$) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H$_2$CO is 0.5$-1.2\times10^{-9}$, which is consistent with results from lower frequency H$_2$CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50 K to values in excess of 100 K. While a systematic trend of (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature ($T_{\rm kin}$ $>$ 100 K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65$\pm$10 K. The high temperatures of molecular clouds over large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs).
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http://arxiv.org/abs/1211.7142
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